Expanding or enlarging digital images is common practice and many techniques to achieve digital image expansion have been considered. As is known, during expansion of an input digital image, it is necessary to generate new pixels between the existing source pixels of the input digital image. One common technique used when an input digital image is being doubled in size, involves determining pixels of the output digital image whose centers coincide with the centers of source pixels in the input digital image and assigning values to those output pixels so that they are simply copies of the source pixels. Pixels in the output digital image whose centers do not coincide with the centers of source pixels in the input digital image are generated as a function of surrounding pixels in the input digital image. Functions used to generate such output pixels include for example linear and cubic interpolations. Unfortunately using linear and cubic interpolation techniques to generate only some of the pixels of the output digital image from source pixels of the input digital image can yield an output digital image that appears jagged and unsmooth.
Other techniques for enlarging a digital image have also been considered. For example, U.S. Pat. No. 5,721,793 to Ushida et al. discloses an image processing method for converting a binary image to a higher density image wherein each pixel in the input image is replaced with four pixels in the output image. During generation of the pixels in the output image, patterns in the input image are used to determine the direction of interpolation. Once the direction of interpolation is determined, the determined interpolation direction is rotated through 360 degrees to determine the various pixels in the input image to be interpolated in order to generate corresponding pixels in the output image.
U.S. Pat. No. 5,754,710 to Sekine et al. discloses an image resolution conversion method for converting an image of a first resolution into an image of a second resolution by interpolation. During the process, a block defined by a plurality of pre-conversion pixels of the first resolution is assigned to each post-conversion pixel. Edge detection in the block based on data for each pixel of the block is then performed. Resolution conversion using a first interpolation method is performed when the result of the edge detection satisfies predetermined conditions. Resolution conversion is performed using a second interpolation method when the result of the edge detection does not satisfy the predetermined conditions.
U.S. Pat. No. 6,608,699 to Kishimoto discloses a video processing apparatus including edge detecting means for detecting an edge direction in an input image, coefficient selecting means for selecting a coefficient based on the edge direction detected by the edge detecting means and filter means for filtering a frequency band using a frequency characteristic corresponding to the coefficient selected by the coefficient selecting means. Edge detections are used to determine the pixels in the input image that are to be interpolated to form the pixels in the output image.
U.S. Pat. No. 6,624,825 to Fujiyoshi discloses a pixel resolution converting circuit to enlarge an input image. A three-by-three pixel array pattern of the input image including the pixel to be converted is compared with specific array patterns formed by possible combinations of pixel bindery image data. If the input image data matches one of the specific array patterns, image data differing from that of the pixel to be converted is applied to the portion of the image data corresponding to four pixels obtained by vertically and horizontally doubling the pixel to be converted.
U.S. Patent Application Publication No. US 2003/0099410 to Daigi discloses an image processing method wherein pixels of an input image are used to generate pixels in an output image by pattern matching. Three-by-three blocks of pixels centered about the pixels of interest in the input image are compared with patterns to determine the pixels to be interpolated, thereby to generate the pixels in the output image.
Although the above references disclose techniques for expanding an image, improvements are desired. It is therefore an object of the present invention to provide a novel method, apparatus and computer program for expanding a source pixel in a digital image and to a method and apparatus for doubling the size of a digital image.